1. Field of the Invention
The present invention relates to a position measuring device.
2. Description of the Related Art
In the course of chip production by wafer steppers it is necessary to position the mask and the substrate extremely accurately. For this purpose it is known to detect positional changes of the mask table by laser interferometers. In this connection the effects of disturbances in the air are detrimental, which in the end lead to positional noise and problems in regard to reproducibility. In order to keep the effects of heat expansion low, the measurement of the position of the substrate table or the mask table directly and relative to the optical imaging system would be advantageous. However, often the mounting of laser interferometers directly on the optical imaging system of the wafer stepper is not possible and not desired for reasons of space limitation and because of the thermal dissipation, so that it is necessary to fasten all parts of the laser interferometer on a special frame made of Invar or Zerodur. The changing of the lasers and the recalibration of the laser interferometer also cause considerable problems. The high cost of the six to ten required interferometer shafts are a further disadvantage.
Position measuring devices in the form of grating measuring systems, which scan a grating scale by an optical measuring principle and provide greater reproduction capabilities are conceivable in place of the elaborate and expensive interferometer techniques for detecting positional changes in the x- and y-directions. For example, such a system is described in the publication “Maβarbeit-Nanometergenaue Positionsmessung in allen Freiheitsgraden” (Precision Work-Position Measuring Correct to a Nanometer in all Degrees of Freedom), Y.-B. P. Kwan et al., F & M Vol. 108 (2000) 9, pp. 60 to 64, which includes one or two cross grating graduations and one or several scanning heads, which detect movements in the x- and y-directions. In this case the cross grating graduations have been applied directly to the mask holder.
In order to meet the particularly high demands made on positioning accuracy and reproducibility in connection with such applications, position measuring devices with very short signal periods (≦500 nm) and many interpolating steps are required. At the same time it is necessary to take into consideration the fact that in the course of aligning the mask with the focal plane of the lens, rotation of the mask holder in all three spatial directions occurs, so that the position measuring device must provide rotatory tolerances between approximately ±3 to ±5 mrad in relation to all three spatial directions. In addition, the position measuring device is required to have a large scanning distance of approximately 5 mm to 20 mm, and to provide a comparatively large distance tolerance of ±1 mm.
A position measuring system of Applicant is known from EP 0 387 520 B1, wherein signal periods of 128 nm result in connection with selected graduation periods, or grating constants of 512 nm. It is possible to achieve positioning accuracies in the sub-nanometer range with this. However, with such small grating constants, twisting of the scale around the direction of the normal line, called Moiré rotation in what follows, leads to opposite directional components along the line direction of the grating graduation of the beams diffracted at the scale. Because of this, the phase surfaces of the interfering signal beams are tilted with respect to each other, which would result in interference strips and a strong signal drop along with it. For solving this problem a triple prism is used as a retro-reflector, which inverts the directional components along the line direction of the grating graduation and in this way images the graduation onto itself. The compensation of the directional components caused by the Moiré rotation will be called Moiré compensation in what follows. A disadvantage of this scanning principle is the beam path, which is inclined in the line direction, i.e. transversely to the measuring direction. The result of this is that distance changes between the scanning unit and the scale cannot be neglected and lead to a change of the indicated position, along with a simultaneous Moiré rotation. Moreover, no large distance tolerances can be achieved by the inclined installation of the scanning unit. A further problem is that the so-called neutral point of rotation does not lie on the scale surface, but in the scanning grating.
By its definition, the neutral point of rotation is understood to be the point around which the scanning unit can be tilted in the measuring direction—this will be called pitch tilt in what follows—without the indicated position being changed in the process. If the neutral point of rotation does not lie on the scale surface, tilting of the scale results in large displacements of the indicated position, which must be eliminated by elaborate correction methods.
Thus, the large prerequisites in positioning the mask table require a position measuring device wherein, on the one hand, the neutral point of rotation is located on the scale graduation, and Moiré rotations are additionally compensated. Moreover, the scale is to be vertically illuminated in order to assure a high degree of symmetry which, for one, permits large distance tolerances, and also avoids other problems in connection with the position determination in case of a distance change and simultaneous Moiré rotation of the scanning unit.